The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop. In addition, white residues are aesthetically unappealing and, thus, are unappealing to both the assembler and their customers.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to remove metal oxides from the surface being soldered to ensure firm, uniform bonding of the solder. These soldering fluxes fall into three broad categories: rosin and water soluble fluxes and no-clean. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
The removal of rosin flux from printed circuit boards has traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment are believed to deplete the ozone layer and/or contribute to the inventory of greenhouse gases. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
The present assignee has developed an effective patented aqueous cleaner marketed under the tradename Armakleen.TM. to remove rosin flux.
However, in order to avoid the use of the chlorinated or hydrocarbon solvents generally employed to remove rosin flux residues subsequent to soldering printed circuit boards, there has been considerable increase in the employment of aqueous type solder fluxes which can be washed subsequent to the soldering operation by means of aqueous detergent solutions or pure water. It has been found, however, that when using water only or aqueous detergent cleaners to remove the aqueous-based solder flux residue, certain contaminants still remain which can result in an increased leakage current in the circuit devices. This problem is particularly acute when silicone type encapsulants are employed for encapsulating the integrated circuit devices. Such encapsulants are generally relevantly soft and have relatively low density and the contaminants become trapped within the encapsulant layer. These contaminants may then eventually migrate to the surface of the device itself resulting inasmuch as an order of magnitude higher leakage current as compared to a control circuit which has not undergone the solder and detergent cleaner procedure. Not only is this higher leakage current undesirable, but the reliability of the devices may be diminished. Similarly, the effectiveness of water and aqueous detergent systems is considerably lacking for cleaning low standoff chip carriers wherein the circuit devices are placed with only minute clearances from the boards as the contaminants become entrapped in the minute clearance space rendering it difficult for any cleaning solution to remove all the contamination.
It has also been found when removing the aqueous-based fluxes with water or detergent compositions and the like, a "white residue" often remains on the surface. The white residue was believed to be primarily the result of the soldering process when using an aqueous fluxing agent. Excessive preheat and soldering temperatures can cause oxidation and decomposition of organic acids used in water soluble fluxes. There are, however, many other possible sources of white residue. The white residue is objectionable, due not only to the appearance, but also the composition is unknown and this raises a concern that the residue may somehow be a corrosion product or be corrosive to the soldered joints. Attempts have been made over the years to remove the residue, all with limited or no success. For example, detergents and surfactants have variously been employed typically in hot water solutions.
U.S. Pat. No. 4,604,144 discloses a method of cleaning circuit boards to remove contaminants which remain subsequent to the detergent cleaning of aqueous-based flux residues on circuit boards. According to the patent, the boards are rinsed in a low molecular weight mono or dicarboxylic acid such as formic, acetic or oxalic acids followed by a thorough rinsing with deionized water to remove the acid. In a preferred embodiment, the organic acid aqueous rinse solution contains a certain amount of an alcohol such as methanol or a soluble freon type fluorocarbon solvent solution. Inasmuch as the use of aqueous-based fluxes were devised so as to eliminate the use of volatile solvents during cleaning of the flux residues, the need for additional alcohol or fluorocarbon solvents as disclosed in this patent is not very desirable.
U.S. Pat. No. 5,312,027 prevents the formation of white residue by exposing the circuit board to a dilute aqueous basic solution prior to a final water rinse. The aqueous basic solution typically comprises a base such as ammonia or sodium bicarbonate.
To clean the aqueous-based flux residues from the circuit board, hot water solutions with or without alkaline detergents and/or surfactants have been utilized. Typically, temperatures at or above 140.degree. F. have been used to clean the water soluble fluxes. Such high temperatures are disadvantageous due to the higher energy costs which are involved relative to lower temperature washing and the difficulty with maintaining these high temperatures. Unfortunately, at lower temperatures, i.e., below 140.degree. F., the desired removal of the aqueous-based flux and other contaminants such as white residues has been even more difficult to achieve.
Accordingly, there is a need to provide a cleaning composition and method which removes not only the aqueous-based soldering flux contaminants but, also other contaminants such as white residue which tend to be left on the integrated circuit devices when employing prior art aqueous detergent cleaning compositions. Additionally, there are distinct advantages to accomplishing this at recovered wash temperatures, i.e., below 140.degree. F.).
There is also a need to provide adequate cleaning of water soluble fluxes and other contaminants from the circuit assemblies using aqueous-based compositions which do not contain volatile components and which are effective at relatively low temperature.
It would also be desirable to provide an aqueous-based cleaner for removing water soluble flux residues and other contaminants from integrated circuit assemblies which are useful at lower temperature and effective to remove any type of water soluble flux. It would be particularly beneficial to remove residues from under components with low standoff heights.
Accordingly, the objectives of the present invention are to provide a cleaning composition for removing aqueous-based fluxes from circuit assemblies which will meet the unmet needs described above. Other objects of the invention will become readily apparent upon a review of the description of the invention and preferred embodiments which are set forth below.